The neurotransmitter of the sympathetic system is norepinephrine (NE) which in response to a stimulus, is released to the synaptic space Guanethidine is a false neurotransmitter analog of NE that is not catabolized . When labeled with radioactive iodine the radiotracer meta-iodobenzylguanidine (as I-MIBG) may be used for localization of different clinical and pathological conditions. There are two main indications for scintigrpahy with labeled MIBG 1. Whole body imaging of neuroendocrine tumors such as phechromocytoma and neuroblastoma. Scintigraphy allows staging of the disease as well as follow up in patients with MIBG-avid tumors. Moreover, when labeled with high doses of 131I it can be used for treatment in patients with metastatic neuroendocrine tumors. 2. the other indication of labeled MIBG scintigraphy is for assessment of myocardial sympathetic nerve endings Autonomic disturbances are of the common symptoms in Parkinson disease (PD). They may appear early in the disease occasionally prior to the motor symptoms, thus raising a potential use of autonomic system assessment as a biological marker for prediction of future development of motor PD.